Stability of superconductors for poloidal field coil application

Experiments were performed on a Cu/CuNi/NbTi mixed-matrix conductor which was supplied with a transport current and exposed to magnetic-field pulses of typically ten milliseconds length and with a dB/dt up to a few hundred tesla per second. Various cooling regimes were employed: bath cooling, supercritical helium, and liquid helium in a closed, small volume (the case of a cable in conduit conductor). The stability limit was found to be determined mainly by the transient heat transfer during the pulse. A stability model is developed which compares the energy coupled into the conductor by the field pulse with the energy which can be absorbed by the near-surface helium layer due to the transient heat transfer. The model assumes that the stability for a short heat pulse is determined by the transient heat transfer during the pulse. The description of transient heat transfer is based on diffusion models; no convective effects are considered. The relevant quantity describing the disturbance is energy input per unit cooled conductor surface. The model is derived for a uniform heat input during a pulse time (rectangular heat pulse), for different cooling conditions.